As is known, according to a solution that is currently very widespread in the micro-electronics industry, the substrate of integrated devices is obtained from monocrystalline silicon wafers. In recent years, as an alternative to wafers of only silicon, composite wafers have been proposed, namely the so-called silicon-on-insulator (SOI) wafers, consisting of two silicon layers, one of which is thinner than the other, separated by a silicon dioxide layer (see, for example, the article “Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations” by J. Hausman, G. A. Spierings, U. K. P. Bierman, and J. A. Pals, Japanese Journal of Applied Physics, Vol. 28, No. 8, August 1989, pp. 1426-1443).
Considerable attention has recently been directed to SOI wafers, since integrated circuits that have a substrate formed starting from such wafers afford considerable advantages as compared to the same circuits formed on traditional substrates of monocrystalline silicon alone.
A typical process for manufacturing SOI wafers is described in the above-mentioned article and is based upon bonding of two monocrystalline silicon wafers (wafer bonding process). The wafers obtained using the traditional wafer bonding method present excellent electrical characteristics but have decidedly high costs (approximately six times the cost of standard substrates).
Other methodologies, such as ZMR, SIMOX, etc., are described in the article “SOI Technologies: Their Past, Present and Future”, by J. Haisha, Journal de Physique, Colloque C4, Supplement au n° 9, Tome 49, September 1988. ZMR techniques have, on the other hand, not yet reached an acceptable industrialization level and present some limitations. In fact, they do not enable monocrystalline silicon layers to be obtained on extensive oxide areas, present a high number of defects on account of the dislocations generated by the stresses induced by the buried oxide, or do not enable high voltages to be reached, for example, in the SIMOX technology, where the oxide thickness obtained by oxygen implantation is about 100-200 nm. Furthermore, SIMOX technology involves a relatively high number of defects, which may give rise to problems during the subsequent processes for integrating the components.
Other processes taught by the applicant (see, for example, European Patent applications EP-A-0 929 095 and EP-A-1 073 112) enable the fabrication of substrates which have costs that are compatible with those of standard substrates but which, however, may be simplified.